White Rust on Galvanized Steel

Zinc Passivation
Like many metals, the good corrosion properties
of zinc are due to a thin film of corrosion
product that forms on the surface of the metal.
This tightly-adherent corrosion film is called a
passivation layer. The formation of the
passivation layer is essential to achieving longlife
with a galvanized product.
Zinc will naturally form a passivation layer
when exposed to weathering over an extended
period of time. Rain combined with air drying
will, over the course of a year or more form a
passivation layer on all areas of the zinc which
do not come in contact with recirculating water.
Thus, the exterior of evaporative equipment and
all interior areas above the drift eliminators will
naturally develop a passivation film.
For the components of the system that are
exposed to or immersed in recirculating water,
the water chemistry must be carefully controlled
until a passivation film forms. Once the film
forms, the recirculating water chemistry, though
still important, is less critical for zinc corrosion.
If the passivation film is damaged by abrasion or
chemical attack, it must be re-formed by again
carefully controlling the recirculating water
chemistry.
It is very difficult to control these water
parameters and form a passivation layer in a
system started under load due to the higher
temperature and the continually changing water
chemistry. Therefore, if possible, the equipment
should be commissioned with water circulation
and without a heat load. The critical parameters
to form the passivation film are described in the
following table. It generally takes 4 to 12 weeks
to passivate zinc with pH control the most
critical parameter and with the first few weeks
of water exposure being the most critical time.
Figure 2
Bright and Shiny
Fresh Zinc
Parameter Passivation Value
pH 7.0 – 8.0
Conductivity 50 ppm
Chlorides